Of moss and micro-arthropods

Mosses were among the first plants to colonize land over 400 million years ago, but they never really left the water behind. Unlike seed plants, mosses (as well as ferns and liverworts) don’t enclose their sperm in pollen but release it directly. Since their unprotected sperm is vulnerable to drying out, these plants can only reproduce if there’s a watery film through which the sperm can swim. However, recent research has started to uncover a more complicated story behind the sex life of mosses. Mounting evidence suggests that mosses recruit micro-arthropods like springtails and mites to carry their sperm in the same way that flowering plants attract and reward insect pollinators.

With over 100,000 individuals in a cubic meter of soil, springtails may be the most abundant animals that are visible to the naked eye. Their name comes from a unique and peculiar mode of escape: a special forked appendage, called a furcula, is kept under tension beneath their abdomen like a tightly wound spring and released when they are threatened, flinging them off to safety. When they’re not busy flicking themselves away from danger, springtails feed on microbes and detritus, often also transporting fungal spores around the soil and vegetation where they live. In 2006, a team of scientists at Lund University in Sweden published a paper in Science showing that springtails can carry sperm between male and female moss plants. They grew the moss in plastic containers, placing the male and female plants slightly apart from each other on a base of plaster; the plaster created a barrier to sperm movement by absorbing any water through which sperm could swim. Some of the containers also included animals, either springtails or moss mites. After three months, only the moss that was grown with animals had successfully reproduced. In another test, they allowed the animals to choose between fertile and sterile moss and discovered that both springtails and mites preferred fertile moss.

As exciting as these results are, they only show that it’s possible for springtails to carry sperm; the paper doesn’t give any evidence that this actually happens in the wild. In another study reported this year in Nature, a group of scientists at Portland State University lend further credence to the importance of this interaction outside the lab. The team, led by Dr. Sarah Eppley, examined the chemicals given off by male and female mosses to find out if they were attractive to springtails. They found that female mosses not only produce more kinds of volatile chemicals than males, but also produce chemicals which are much more different from each other. Intriguingly, many of the compounds in this rich bouquet are already known from floral scents. The study also showed that springtails prefer female over male moss and that this preference is thanks to the scent, since the preference remains when the animals can only smell the plants without seeing or touching them. Finally, they grew moss under more natural conditions than were used in the first study and confirmed that springtails still increased the rate of fertilization — even if they plants were also sprayed with water.

These papers clearly show that springtails can transport moss sperm and that this improves the rate of fertilization. Despite all of this excellent work, the results are still only indirect evidence. As I read these papers, I wondered: why not simply go and check directly? Why not collect a bunch of springtails from the field and find out if they really are covered with moss sperm? I put the question to Dr. Eppley. “Needless to say, there are several labs working on this,” she replied, “but it is rather tricky in some regards.” Researchers would have to demonstrate that the sperm is actually from moss and that it was still viable. “Pollen has been easier to work with in this regard,” said Dr. Eppley.

Even though the process hasn’t yet been observed in the wild, these papers make a strong case that micro-arthropods play an important role in moss reproduction. Flowering plants only appeared about 140 million years ago, so this interaction would pre-date the evolution of pollination and could have major implications for the ecology and evolutionary history of mosses and other plants. In this era of high-throughput, big-data oriented science, it’s refreshing to see such disarmingly simple studies published in top journals like Nature and Science. It’s a reminder of how much we still have to discover about the rich world around us and the fact that great science doesn’t have to be about huge projects: sometimes, all it takes is asking the right questions about something going on right under your feet.